You may have to dispute this account as "Not Mine" on all 3 CRs. I've read here recently of someone else having a similar problem with getting Discover AU account removed from their credit reports and I believe this is what they had to do.
I posted elsewhere about my experience as a Discover AU and unfortunately, it's not very encouraging.
When I called Discover to advise TL was still showing on my report in April even though I was deleted as AU in January, I was advised the TL would report UP TO the date of deletion.
Because it was still reported, the balance and the account age was being figured into my FICO score, which was detrimental. I disputed with the CRAs as "not mine" explaining in comments that I was deleted as AU.
TU deleted TL completely, EX and EQ verified but at least corrected to report the TL with 0 balance. EQ shows "account relationship terminated" as a comment. The 0 balance at least helped utilization, but the account being newer (07-08) is not helping my FICO and I don't think EQ's comment sounds very positive either.
While I love my Discover Card (I do have my own) and their service has been great, their procedure for AUs is more difficult than others. BoA and Citi took me right off DH's card and the TLs right off the CR.
Deleting the TL just doesnt seem to be the way Discover reports the deletion of AU from an account, and the fact that TU deleted it from mine after dispute is proably just a fluke. Maybe Discover handles AU this way to discourage those unscrupulous ones that were paying to be added to accounts etc, I don't know.
I'm assuming there's nothing else I can do, but if anyone can chime in about personal experience with this, please do!!
Where do I go to dispute it?
If you use one of the credit monitoring sites (TC or CCT), you can dispute from their site, except for EQ. I believe you have to pull a report from their site or here to dispute with them.
After reading more here about Discover and TU it seems you may not have such an easy time getting it removed. I've read that they will stop reporting activity on the card but that the TL remains. Hopefully you called to be removed as AU before your mother did the BT. You really don't want it to stop reporting activity with that on the report.